Electrical power supply units



Munch 22, 1960 D. GORMLEY ELECTRICAL POWER SUPPLY UNITS File'd June 17, 1955 ON VH Q R 0% a xi F i u v 2 2 n QR H n Na 1 m R R mm M RR bk \m A tlorn ey United StatesPatent ELECTRICAL POWER SUPPLY UNITS David Gormley, Lnton, England, assignor to General Motors Corporation, Detroit, Micln, a corporation of Delaware Application June 17, 1955, Serial No. 516,161 6 Claims. (Cl. 321-2) This invention relates to a control device for an electrical power supply unit for electrostatic discharge systems suchas those used in spray painting, cleansing of gases by electrostatic precipitation, and detearing electrostatically; that is, whenever close control is desired of the maximum current flowing at high voltage between electrodes producing an electrostatic field. In such systems, particularly those used in spray painting, it is desirable to avoid sparking between the electrodes producing the electrostatic field. The control de vice of the present invention automatically anticipates sparking by cutting off the interelectrode discharge current when it reaches a predetermined value above which sparking would be liable to occur.

According to the invention, a control device for an electrical power supply unit for electrostatic discharge systems comprises a diode arranged for the passage therethrough of a current for producing the interelectrode discharge current, and in parallel therewith an electron dis charge device, preferably a thyratron, and a relay in series with it, so that the anode voltage applied to the electron discharge device is proportional to the diode current, the arrangement being such that only when the diode current exceeds a predetermined value does the electron discharge device become conductive and so operate the relay to cut off the inter-electrode discharge current.

Preferably the relay is adapted to cut off the current in the input circuit of the power supply unit. This is conveniently done through the intermediary of a second relay whose contacts are in said input circuit. The second relay can be arranged to operate a warning signal, which may be visible or audible or both.

The scope of the invention is defined by the appended claims; and how it can be performed is hereinafter particularly described with reference to the accompanying drawing, which is a circuit diagram of a power supply unit incorporating the control device.

The power supply unit has input terminals 10 and 11 for a supply voltage of 250 volts at 50 c./s. The input terminals 10 and 11 are connected to the primary winding 12P of a transformer 13. Between terminal 11 and the primary winding HP is a switch 14 and a normally open relay contact 16. By normally-open is meant open when the associated relay is not energised. Connected across terminals 10 and 11 are a green indicator lamp 17 and a normally-closed relay contact 18 in series; and a red indicator lamp 19 and a normally-open relay contact 20 in series. Also connected across terminals 10 and 11 is a 250 volt relay 21 and a normally-closed relay contact 22. The relay 21 operates contacts 16, 18 and 20.

The transformer 13 has a secondary winding 128 the ends of which are connected to anodes 24 and 25 of a rectifier valve 27 (V4). The centre tap of the secondary winding 12S is earthed at 26. A further secondary winding 28 of transformer 13 supplies cathode voltage for valve 27; and the cathode 29 of valve 27 is connected through a smoothing circuit consisting of earthed con- Patented Mar. 22, 1960 2 densers 30 and 31 and a choke 32 to a control circuit in which is a diode 33 (3524GT). Choke 32 is connected to the anode 34 of diode 33 through a milliammeter 35, and the cathode 36 of diode33 is connected through a tuned circuit consisting of capacitors 37 and 38 and inice ductance 39 to the anodes 40 and 41 of beam tetrode oscillator valves 42 and 43 (6L6) respectively, in the output circuit of the power supply unit.

In parallel with diode 33 is a thyratron 44, the cathode 45 of which is connected through a highly sensitive relay 46, which controls contact 22,'to the cathode 36 of diode 33. The anode 47 of thyratron 44 is connected to the choke 32 and the control grid 48 of thyratron 44 is connected to the movable contact of a variable resistor 49 one end of which is connected to the anode 47 of thyratron 44, and the other end is connected through a fixed resistor 50 to earth. In parallel with diode 33 is a voltmeter 51.

The oscillator valves 42 and 43 have their cathodes 52 and 53 respectively joined and earthed through a common variable resistor 54 and parallel capacitor 55. The control grids 56 and 57 of valves 42 and 43 respectively are connected through resistors 58 and 59 to a coupling inductance 60 which is connected to earth through a resistor 61 and parallel capacitor 62. The screen grids 63 and 64 of valves 42 and 43 respectively are connected through resistors 65 and 66 respectively to the cathode of diode 33. Screen grid 63 is connected through capacitor 67 to earth and screen grid 64 is connected through capacitor 68 to earth.

The inductance 39 is coupled to an inductance 69, the two inductances 39 and 69 forming a high frequency stepup transformer. One end of inductance 69 is earthed through a microammeter 70 and the other end of inductance 69 is connected to the cathode 71 of a rectifier valve 72 (1B3). The anode 73 of rectifier valve 72 is joined to the cathode 74 of a rectifier valve 75 (1B3) the anode 76 of which is in turn joined to the cathode 77 of rectifier valve 78 (1B3). The anode 79 of rectifier valve 78 is connected to one electrode 80 of a two-electrode spray painting system, the other electrode 81 being earthed.

To operate the power supply unit switch 14 is closed, and this causes the green indicator lamp 17 to light. To energise the oscillator valves 42 and 43 the push button 23 is depressed thereby energising relay 21 which closes contacts 16 and 20 and opens contact 18. The green indicator lamp 17 is extinguished and red indicator lamp 19 lights. The closing of contact 16 applies voltage to the transformer 13 and through rectifier 27 and diode 33 to the oscillator valves 42 and 43.

The high frequency oscillatory voltage produced by the oscillator valves 42 and 43 is transformed by the inductances 39 and 69 into a high frequency higher voltage which is applied to the chain of rectifiers 72, 75, 78 and emerges as a negative direct current high voltage (E.H.T. voltage) of 40 kv. which is applied to the electrode 80 and results in the passage of a discharge current between electrodes 80 and 81.

The discharge current and the current producing it, that is, the current flowing through diode 33 of the control device, are interdependent, and if the discharge current changes then the diode current changes in the same sense. The voltage on the control grid 48 of the thyratron 44 is so arranged that the thyratron 44 is non-conductive until a predetermined current flows through the diode 33. The current flowing through the diode 33 sets up a proportional voltage across the thyratron 44 and the arrangement is such that the thyratron 44 conducts only when the current through the diode 33 exceeds a predetermined value. The predetermined value is di rectly related to that value of the discharge current above en a es which sparking would be liable to, occur and is determined by experiment with a given pair of electrodes 80 and 81 under given conditions of operation and may be altered by adjusting the bias on control grid 48.

When the inter-electrode discharge current reaches a value above which sparking would be liable to occur, so that the current through diode 33 exceeds the predetermined value the thyratron 44 strikes, that is to say becomes conductive, and energises relay 46. Energisation of relay 46 causes the normally closed contacts 22 to open thereby deenergising relay 21. ,Relay 46 is a highly sensitive relay and responds very quickly to very small currents: relay 21 is a fast-acting 250 volt relay operating on larger currents.

Deenergisation of relay 21 causes contacts 20 to open thereby extinguishing the red lamp 19 and contacts 18 to close thereby lighting the green lamp 17; and also causes contact 16 to open thereby removing power from transformer 13 and removing the HT. voltage from the anodes of the oscillator valves 42 and 43. Thus the E.H.T. voltage is removed from the spray painting electrodes 80 and 81 and the discharge current between these electrodes is cut off.

To re-set the power unit it is only necessary to depress push-button 23.

The thyratron 44 acts as an automatic switch which is sensitive to an extremely small increase in the total current flowing between electrode 80 and earthened electrode 81 thereby anticipating sparking conditions and switching off the power unit when current between the electrodes exceeds said predetermined value.

The voltmeter and milliammeter and microammeter are inserted to indicate currents and voltages involved near sparking conditions and are not essential to the operation of the power unit.

The relay 21 can be used to operate an audible warning such as a bell or buzzer to indicate that the power unit is no longer operative.

I claim:

1. A power supply unit for an electrostatic discharge system comprising an input circuit adapted to be connected to a source of power and including a rectifier; a diode connected to said rectifier; an electron discharge device in parallel with said diode and adapted to conduct only when the diode current exceeds a predetermined value; an output circuit containing an oscillator connected to said diode, said output circuit being adapted to be connected to discharge electrodes; and a relay in series with said electron discharge device and adapted to be energised when said electron discharge device conducts to cut oil the diode from the source of power.

2. A power supply unit for an electrostatic discharge system comprising an input circuit adapted to be connected to a source of power and including step-up transformer and a rectifier connected to the secondary winding thereof; a diode connected to said rectifier; an electron discharge device in parallel with said diode and adapted to conduct only when the diode current exceeds a-predetermined value; an output circuit containing a thermionic tube high-frequency oscillator connected to said diode, said output circuit being adapted to be connected to discharge electrodes; and a relay in series with said electron discharge device and adapted to be energised when said electron discharge device conducts, to cut oif the diode from the source of power.

3. A power supply unit for an electrostatic discharge system comprising an input circuit adapted to be connected to a source of power and including a rectifier; a diode connected to said rectifier; an electron discharge device in parallel with said diode and adapted to conduct only when the diode current exceeds a predetermined value; an output circuit containing a thermionic tube oscillator connected to said diode, said output circuit being adapted to be connected to discharge electrodes; and a relay in series with said electron discharge device and adapted to be energised when said electron discharge device conducts, to cut off the current in said input circuit.

4. A power supply unit for an electrostatic discharge system comprising an input circuit adapted to be connected to a source of power and including a step-up transformer and a rectifier; a diode connected to said rectifier; an electron discharge device in parallel with said diode and,adapted to conduct only when the diode current exceeds a predetermined value; an output circuit containing a high-frequency oscillator connected to said diode and a rectifier connected to said oscillator, said output circuit being adapted to be connected to discharge electrodes; and a relay in series with said electron discharge device and adapted to be energised when said electron discharge device conducts, to cut ofi the diode from the source of power.

5. A power supply unit for an electrostatic discharge system comprising an input circuit adapted to be connected to a source of power and including a step-up transformer and rectifier; a diode connected to said rectifier; a thyratron in parallel with said diode and adapted to conduct only when the diode current exceeds a pre determined value; an output circuit containing a highfrequency oscillator connected to said diode and a rectifier connected to said oscillator, said output circuit being adapted to be connected to discharge electrodes; a first relay in series with said electron discharge device and adapted to be energised when said electron discharge device conducts, and a second relay controllable by said first relay and adapted to cut off the current in said input circuit when said thyratron conducts.

6. A power supply unit for an electrostatic paint spray discharge system comprising an input circuit adapted to be connected to a source of power and including a step-up transformer and a rectifier; a diode connected to the output side of said rectifier; a thyratron in parallel with said diode and adapted to conduct only when the diode current exceeds a predetermined value; an output circuit containing a high-frequency thermionic tube oscillator connected to said diode, and a rectifier connected to said oscillator, said output circuit being adapted to be connected to discharge electrodes; a first relay in series with said thyratron and adapted to be energised when said electron discharge device conducts, and a second relay controllable by said first relay and adapted to cut ofi current in said input circuit when said thyratron conducts.

References Cited in the file of this patent UNITED STATES PATENTS 2,472,042 Davies May 31, 1949 2,509,277 Rausburg May 30, 1950 2,650,329 Orndoff Aug. 25, 1953 2,815,446 Coombs Dec. 3, 1957 

